A. The integrase proteins of phages lambda and HKO22 are site specific recombinases that are closely related in structure and function, but recognize different nucleotide sequences in the core regions of their substrates, the attachment sites of the two phages. In order to locate the parts of the proteins that discriminate between the substrates, we constructed and analyzed the substrate specificity of a large set of lambda/HKO22 hybrid proteins. These proteins were produced by selecting crossovers between short regions of identical sequence shared by the lambda and HKO22 int genes. We found that exchange of segments upstream of amino acid residue 202 or downstream of residue 283 in lambda integrase had little or no effect on substrate specificity, and we conclude that the 'discrimination domain' lies between these residues. B. We have begun a mutational analysis of E. coli Integration Host Factor (IHF) with the goal of determining which regions of this small DNA binding and bending protein are directly involved in specific sequence recognition and DNA bending. We have so far isolated and characterized a small set of mutations that interfere with the function of the wild type protein ('dominant negative' mutations). We have also found, contrary to previous indications, that IHF is abundant in exponentially growing cells: the concentration is comparable to that of ribosomal proteins. C. We have continued our studies of the action of the HKO22 Nun protein, a sequence specific transcription termination factor. We have shown that partially purified Nun protein specifically strongly and specifically depresses the synthesis of beta-galactosidase in a coupled transcription-translation system, and that Nun action requires the presence of a functional nut site upstream of the lacZ gene. The requirement for a nut site agrees with previous in vivo studies. Surprisingly, the effect of Nun in this system appears to be mainly post-transcriptional: Nun depresses lacZ message much less strongly than beta-galactosidase. We are pursuing this suggestion that Nun may act on more than one level.